Review: Binary Phase Diagrams N0zєXAXBN0zєXAXB є =  AB – ½ (  AA +  BB ) , є > 0   AB is large  avoid A-B bonds  clustering, unmixing, phase separation all solutions: when entropy is only configurational: ideal solutions: regular solutions: regular, positive enthalpy

Regular solutions, positive enthalpy solid phase is a regular solution,  > 0; liquid phase is a an ideal solution Liquid Binary Eutectic ´´  ´´   + L TMATMA TMBTMB  ´+  ´´ liquidus solidus increasing  ´´  ´´   ´+  ´´  + L liquidus solidus TMATMA TMBTMB

Binary Eutectic Phase Diagram For  >> 0, no longer useful to think of A and B as forming a continuous solution T1T1 XBXB XBXB

Binary Eutectic Phase Diagram For  >> 0, no longer useful to think of A and B as forming a continuous solution XBXB XBXB At the eutectic temperature three phases in equilibrium

Regular solutions, negative enthalpy N0zєXAXBN0zєXAXB є =  AB – ½ (  AA +  BB ) , є < 0   AB is small  favor A-B bonds  ordering, compound formation regular solutions: regular, negative enthalpy both terms < 0 and symmetric about X A = 0.5

Regular solutions, negative enthalpy solid phase is a regular solution,  < 0; liquid phase is a an ideal solution  G mix XBXB maximize AB bonds  ‘AB’ compound AB  ´+ AB  ´´+ AB  ´´ ´´ at high T, T  S mix dominates, at low T,  H mix becomes important

Regular solutions, negative enthalpy solid phase is a regular solution,  < 0; liquid phase is a an ideal solution  G mix XBXB AB  ´+ AB  ´´+ AB  ´´ ´´ high T low T ‘AB’ region narrows at high T XBXB liquid L + S(  ) |  | is small so liquid region unaffected  |  |  will impact liquid region TMATMA TMBTMB

Regular solutions, negative enthalpy liquid L + S(  )  TMATMA TMBTMB solid phase is a regular solution,  < 0; liquid phase is a an ideal solution increasing |  | XBXB XBXB T TMATMA TMBTMB AB  ´+ AB  ´´+ AB  ´´ ´´  L + S(  ´) L + S(  ´´) solid with AB (solid) bonds preferred over random liquid also moves up

Regular solutions, negative enthalpy solid phase  << 0, AB compound forms (  ), no longer meaningful treat as a solution; liquid phase is a an ideal solution increasing |  | XBXB T TMATMA TMBTMB AB  ´+ AB  ´´+ AB  ´´ ´´  L + S(  ´) L + S(  ´´) really moves up XBXB TMBTMB TMATMA liquid TMTM

Regular solutions, negative enthalpy solid phase  << 0, AB compound forms (  ), no longer meaningful treat as a solution; liquid phase is a an ideal solution XBXB TMBTMB TMATMA liquid TMTM G XBXB L T1T1 consider , ,  as distinct

Shape of G curves wide composition range, anti-site defects narrow composition range, few defects G narrow

Now consider non-ideal liquid XBXB XBXB  solid and  liquid > 0,  H mix > 0  tendency to cluster (unmix) increasing  Peritectic

Now consider non-ideal liquid XBXB XBXB  solid and  liquid >> 0, A hates B and B hates A,  phase separation even in the liquid or Monotectic

Now consider structure change in solid T L +   +   +   +  Liquid = ideal  solid = ideal  /  – not a continuous solution (or  >> 0) Eutectoid

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